A photovoltaic device generates electrical power by converting light into direct current electricity using semiconductor materials that exhibit the photovoltaic effect. The photovoltaic effect generates electrical power upon exposure to light as photons, packets of energy, are absorbed within the semiconductor to excite electrons to a higher energy state. These excited electrons are thus able to conduct and move freely within the material
A basic unit of photovoltaic device structure, commonly called a cell, may generate only small scale electrical power. Thus, multiple cells may be electrically connected to aggregate the total power generated among the multiple cells within a larger integrated device, called a module or panel. A photovoltaic module may further comprise a protective back layer and encapsulant materials to protect the included cells from environmental factors. Multiple photovoltaic modules or panels can be assembled together to create a photovoltaic system, or array, capable of generating significant electrical power up to levels comparable to other types of utility-scale power plants. In addition to photovoltaic modules, a utility-scale array would further include mounting structures, electrical equipment including inverters, transformers, and other control systems. Considering various levels of device, from individual cell to utility-scale arrays containing a multitude of modules, all such implementations of the photovoltaic effect may contain one or more photovoltaic devices to accomplish the energy conversion.
Thin film photovoltaic devices are typically made of various layers of different materials, each serving a different function, formed on a substrate. A thin film photovoltaic device would include a front electrode and a back electrode to provide electrical access to the photoactive semiconductor layer or layers sandwiched between.
In one example of a photovoltaic device, the substrate would be a glass sheet, such as soda lime glass, float glass, or low iron glass, but could also be a polymer or other suitable material. The substrate may have various surface coatings on the internal and external surfaces, in the context of the finished device. That is, the external surface is exposed to the environment, and the internal surface is encapsulated within the photovoltaic device. The surface coating on the external surface may include an anti-reflective coating, an anti-soiling coating or other coating to improve the device performance. The substrate may include coatings on the internal surface, such as a buffer layer, transparent conductive oxide (TCO) layer and a barrier layer.
The internal coatings together comprise a TCO stack. The barrier layer lessens diffusion of sodium or other contaminants from the substrate to the semiconductor layers. The buffer layer decreases the likelihood of irregularities occurring during the formation of the semiconductor layer or layers. The TCO layer is a transparent, electrically conductive, material serving as a front electrode to the photovoltaic device to communicate a generated electrical current to a circuit, which may include an adjacent photovoltaic device, such as to adjacent cells within a photovoltaic module.
The semiconductor layer or layers will typically include a p-n junction that drives an electrical current as light is absorbed within the material. A p-n junction may be formed by of a bilayer where the first layer is an n-type layer referred to as the window layer and where the second layer is a p-type layer referred to as the absorber layer. When light is incident on the photovoltaic device, photons will excite electrons to a higher energy level causing them to conduct within the material. Front and back electrodes are connected to the semiconductor layer or layers to provide a front and back current pathways to take advantage of the conducting electrons. The efficient operation of the device, that is, how much light energy incident on the device is converted and collected as usable electrical power, may be negatively affected by losses as the generated current flows between adjacent layers of dissimilar materials. These losses may include resistance losses, and may also include loses due to recombination of mobile charge carriers.
The manufacturing of a photovoltaic structure generally includes sequentially forming the functional layers through a process that may include vapor transport deposition, atomic layer deposition, chemical bath deposition, sputtering, closed space sublimation, or any other suitable process that creates the desired material. Once a layer is formed it may be desirable to modify the physical characteristics of the layer through subsequent treatments processes. For example, a treatment process step may include passivation, which is defect repair of the crystalline grain structure, and may further include annealing. Imperfections or defects in the crystalline grain of the material disrupt the periodic structure in the layer and can create areas of high resistance or undesirable current pathways, for example, parallel to but separated from the desired current pathway such as a shunt path or short.
An activation process may accomplish passivation through the introduction of a chemical dopant to the semiconductor bi-layer as a bathing solution, spray, or vapor. Subsequently annealing the layer in the presence of the chemical dopant at an elevated temperature provides grain growth and incorporation of the dopant into the layer. The larger grain size reduces the resistivity of the layer, allowing the charge carriers to flow more efficiently. The incorporation of a chemical dopant may also make the regions of the bi-layer more n-type or more p-type and able to generate higher quantities of mobile charge carriers. Each of these improves efficiency by increasing the maximum voltage the device can produce and reducing unwanted electrically-conductive regions. In the activation process, the parameters of anneal temperature, chemical bath composition, and soak time, for a particular layer depend on that layer's material.
Therefore, it is desirable to provide an effective back electrical connection at the back of the photovoltaic device to minimize losses that may occur at the interface between the absorber layer and the back current pathway and a method of making such a photovoltaic device.